[Request] Add conf.lua option to enable touchpad touch events on macOS

[turbo]

On Apple computers with Magic Trackpads (MacBooks, and the standalone Magic Trackpad product), the surface is actually a multi-touch pad. This is important for implementations of e.g. natural scrolling, which depends on access to touch events themselves. E.g. on any scrollable surface on macOS, you can initiate a scroll with inertia, lift off your fingers, and then stop it dead by simply touching again (just an example, there are a lot more gestures). I'd like to make use of some of the gestures in Love, but currently MTs don't register touch events - as far as I can tell, please correct me if I'm wrong!

SDL2 has solved this problem a while ago with this hint: https://github.com/love2d/love-apple-dependencies/blob/b3397c1464537d7ad418a1d60ffced71fc919cd4/macOS/Frameworks/SDL2.framework/Versions/A/Headers/SDL_hints.h#L2442-L2461

I'd like to propose adding a config option that allows settings this hint from a game's config.lua. SDL has indicated the same hint will apply to future touchpad support beyond macOS, but it is specific to Cocoa right now.

I tried to add this to the latest Love source, but I'm not sure where the right place is. The hint docs indicate it should be called before video init, so I added it to Window.cpp before the init call, but it didn't do anything. To test, I used the default touch example from the Wiki:

// in the Window constructor, before SDL_InitSubSystem

#ifdef LOVE_MACOS
 /**
  * The variable can be set to the following values:
  *
  * - "0": Trackpad will send mouse events. (default)
  * - "1": Trackpad will send touch events.
  *
  * This hint should be set before SDL is initialized.
  */
 SDL_SetHint(SDL_HINT_TRACKPAD_IS_TOUCH_ONLY, "1");
#endif

[turbo]

Hm, looking a bit more into the SDL source, I'm not sure if this is fully implemented after all. But that's a judgement beyond my understanding of SDL internals.

[turbo]

I gave up on my SDL investigation. For anyone stumbling upon this, I solved this for me by compiling this code which talks directly to MultitouchSupport (a system Framework) to a C library and then using it from Lua. This doesn't just give the position of touches, but also the velocity and even the rotation (which is very accurate!) for each finger.